I want to use a firewall interconnector for the EGT Sensors. As I understood there is built in cold junction compensation in the ECU.
I would use normal copper wires (shielded MIL 27500) from the ECU to the connector thus moving the cold junction to the firewall and potentially creating an offset in the measurements.
Can anyone quanity the amount of error I have to expect from such a setup? Of course this depends on the temperature difference. für Instance, ECU at 30° and firewall connector at 80°
I know that there are special pins for the connectors that should be used, but they cost too much in my opinion.
I am planning on using the EGTs for individual cylinder trim and injector fault safety. I think having a small offset on all the EGT sensors wont be an issue for this use case.
If ECUmaster implements the use of an external Sensor as the source for the cold junction compensation temperature how would I need to set this up in Hardware?
Should I prepare a thermocouple in the Connector or should I prepare a non termocouple sensor?
I am about to start building this subharness and would like to prepare everything so that I could enable such a compensation in the future or calculate my own via the project tree.
The error introduced is approximately equal to the thermocouple voltage corresponding to the temperature difference between the actual cold junction and the assumed one.
Let’s assume:
Thermocouple type: K-type (common for EGT)
Temperature difference: 80 °C (firewall) – 30 °C (ECU) = 50 °C
For a K-type thermocouple:
The voltage change per °C is roughly 41 µV/°C
So, the error voltage = 50×41=2050 μV=2.05 mV50×41=2050 μV=2.05 mV
This corresponds to a temperature error of about 50 °C in the EGT reading — quite significant!
To implement this feature, we would probably add a new dedicated sensor called something like “Cold junction temperature”. You would be able to configure this sensor to use either a thermistor, a thermocouple, or a custom value, such as one from a CAN bus.
Then we would need an option for each connected thermocouple to use either the internal or the external cold junction temperature. Internal is the one measured by the ECU at the connector, and external would be the predefined temperature sensor.
Cold junction temperature is basically just an offset. If your cold junction compensation is off by 15 degrees, the overall reading is also off by 15 degrees.
To sum up, you need any temperature sensor that can be read by EMU, and mount it to measure the temperature of the connector or firewall around it.
Nice, that would be exactly what is needed.
The option to choose the internal or external sensor was the one thing I was not sure about, but in this case everything is clear and I am going to integrate a thermocouple inside the connector to have this option.
I think I can implement my own offset correction in the project tree in the meantime.
Ill look further into when I am back at the software. Add the temp difference between ECU temp and the temp at the firewall to the EGT readings, create new variables and use them instead on all the EGT correction maps…